In a modern passenger aircraft the passenger cabin or sub-regions of the passenger cabin, the hold or sub-regions of the hold, the cockpit and the crew restrooms form various air-conditioning zones that are conventionally air-conditioned both during cruising and during operation of the aircraft on the ground by means of an onboard air-conditioning system. The aircraft air-conditioning system is supplied with hot bleed air under increased pressure that is removed from the engine compressors or auxiliary engine compressors. In the air-conditioning units, the so-called air-conditioning packs of the aircraft air-conditioning system, the bleed air is expanded and is cooled to a desired low temperature. Finally, the air conditioned in the air-conditioning units is directed as cooled fresh air into a main fresh-air line of the air-conditioning system. The fresh air flowing through the main fresh-air line is conveyed into a mixer, where it is mixed with recirculated air extracted from the passenger cabin. The mixed air produced in the mixer from cold fresh air provided by the air-conditioning packs and from recirculated air extracted from the aircraft cabin is finally used to air-condition the various air-conditioning zones of the aircraft.
In order to allow individual air-conditioning zones of the aircraft, for example a hold region or the cockpit, to be air-conditioned individually independently of recirculated air, fresh air has to be removed upstream of the mixer from the main fresh-air line of the aircraft air-conditioning system. Conventionally, a fresh-air branch line that branches off upstream of the mixer from the main fresh-air line is used for this purpose. In a fresh-air branch line branching off from the main fresh-air line, however, regions where a fresh air branch flow directed through the fresh-air branch line has a reduced flow rate usually occur. As a result, in the event of a corresponding temperature and a corresponding moisture content of the fresh air, the attachment of ice and snow to the lines of the air-conditioning system is promoted in the region where the fresh-air branch line branches off from the main fresh-air line. The smaller the cross section of flow of the fresh-air branch line, the greater the risk of icing in the region where the fresh-air branch line branches off from the main fresh-air line.
From DE 10 2006 037 539 A1 it is known for fresh-air lines, through which cold fresh air from the air-conditioning units of an aircraft air-conditioning system flows, to be protected against icing by means of a supply of warm engine bleed air that is controlled by trimming valves. However, as a result of the supply of warm engine bleed air into the fresh-air lines the cooling capacity of the air-conditioning system is reduced. Furthermore, the trimming valves needed to control the supply of engine bleed air into the fresh-air lines are relatively susceptible to faults. This leads to an increased outlay for maintenance as a failure of the trimming valves would result in impairment of the operation of the aircraft.